1. Field of the Invention
The present invention relates generally to belt drives, more specifically to hard copy apparatus having a scanning carriage for translating writing instruments across print media and, more particularly, to a thermal compensation belt drive tensioner for a scanning ink-jet printer carriage.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed for example, in various articles in the assignee""s Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press. San Diego, 1988).
FIG. 1 depicts a hard copy apparatus, in this exemplary embodiment a computer peripheral, ink-jet printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller 102 (usually a microprocessor or application specific integrated circuit (xe2x80x9cASICxe2x80x9d) controlled printed circuit board) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC""s. Cut-sheet print media 105, loaded by the end-user onto an input tray 120, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or xe2x80x9cprint zone,xe2x80x9d 123 where graphical images or alphanumeric text is created. A carriage 109, selectively positionable on a rod 111, scans the print medium. An encoder subsystem 113 is provided for keeping track of the position of the carriage 109 at any given time. A set of individual ink-jet pens, or print cartridges, 115X is mounted in the carriage 109 (generally, in a full color system, inks for the subtractive primary colors, cyan (X=C), yellow (X=Y), magenta (X=M) and true black (X=K) are provided; in some implementations an ink-fixer chemical (X=F) is also used). An associated set of replaceable or refillable ink reservoirs 117X is coupled to the pen set by ink conduits 119. Once a printed page is completed, the print medium is ejected onto an output tray 121. The carriage scanning axis is conventionally designated the x-axis, the print media transit axis is designated the y-axis, and the printhead firing direction is designated the z-axis. For convenience in describing the art and the present invention, all types of ink-jet hard copy apparatus are sometimes hereinafter referred to as xe2x80x9cprinters;xe2x80x9d all types, sizes, and compositions of print media are also referred to as xe2x80x9cpaper;xe2x80x9d all compositions of colorants are sometimes referred to as xe2x80x9cink;xe2x80x9d and all embodiments of an ink-jet writing instruments are sometimes hereinafter simply referred to as a xe2x80x9copen;xe2x80x9d no limitation on the scope of the invention is intended nor should any be implied.
Generally, a belt drive subsystem can be used to mount and selectively move the carriage 109 for scanning bidirectionally across the print zone 123. Instantaneous positioning of the printhead to the print medium is critical to prevent a printing error and ensure throughput performance. Belt tension is an important parameter and is varied dependent primarily on the drive loads required, margin desired, and belt-pulley design.
FIG. 2 (PRIOR ART) illustrates a belt 100 connected to a reversible motor 300 and a pulley 500, both of which are usually affixed to a printer housing framework (not shown). A pulley axle 700 fits slidingly in slots 900 in a mounting frame 110. Assembled, the belt 100 extends through an aperture 130 in the frame 110. The pen carriage 109 (not seen in this view) rides on a guide rod 111 as in FIG. 1 and is attached to the belt 100. Since the pulley axle 700 makes a sliding fit and the belt must be long enough to reach beyond the end 170 of the mount 110 to encircle the pulley 500 before it is fit into the slots 900, a spring loaded tensioner 190 is used to achieve the proper post-assembly tension. After the pulley 500 is fit into the slots 900, the tensioner 190 is inserted such that the axle 700 will ride on a tensioner surface 210. An extension table 230, with a tensioning spring 250 surrounding it, is rotatingly slipped into slot 270 of the mount 110. By designing to a close tolerance, the tensioner surface 210 will pull the pulley outward along the slots 900 just until the belt 100 is properly tensioned. Another spring loaded tensioner is shown in U.S. Pat. No. 4,761,154, filed by Beauchamp et al. for a BELT TENSIONER (assigned to the common assignee herein and incorporated herein by reference).
Another prior art design, that has a lower belt tension requirement over the spring loaded design, is the standard fixed center tensioner that includes a spring loaded belt tensioner that provides initial belt tension and a locking mechanism to fix the tensioner to a predetermined setting. As the belt stretches under load and during thermal and humidity excursions, the hardware expands and contracts, resulting in changing pulley center-to-center spacing. The result is a change in belt tension from the initial setting.
In such solutions, as the belt stretches under load and during thermal and humidity excursions, the hardware can also expand and contract, resulting in a spring deflection and, therefore, a belt tension change. Such systems are designed with the lowest possible spring rate so that such a tension change is minimized. However, at higher acceleration and deceleration loading and higher scanning speeds which increase printing throughput, belt tensions must be increased to prevent belt slip at the motor and pulley interfaces. Moreover, for an ink-jet implementation, due to the advancement in pen design and increasing the number of printheads on-axis (or, for disposable or refillable print cartridges having a self-contained, on-axis, ink supply where the cartridge size is increasing to meet the demand for full bleed printing (e.g., photographic reproductions)), the total carriage weight increases. Increasing belt tension increases loading on the motor axle bearings. One solution is to add precision bearings to the motor shaft, but only at a significant cost impact on manufacturing. During temperature and humidity excursions, the material will experience an expansion or contraction (xe2x80x9cECxe2x80x9d) proportional to the respective material coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d). Note that CTE can be a positive or negative value. Depending upon the materials used in fabricating the mechanism, differing material CTE may generate a belt tension increase or decrease during temperature excursions.
There is a need for a method and apparatus to maintain a substantially constant belt tension during temperature changes, There is a need for a method and apparatus to run relatively low belt tensions in high speed printers to increase motor life. There is also a need for a low cost solution.
In its basic aspects, the present invention provides a belt tensioner device, including: an adjustably mountable frame for mounting to a chassis; a pulley fork biasingly mounted to the frame; and a belt pulley mounted between the frame and the fork wherein the frame and fork are co-associated and fabricated of materials each having a complementary CTE to compensate for temperature excursions affecting belt tension.
In another basic aspect, the present invention provides a belt tensioner for an apparatus having a chassis with a belt drive motor affixed thereon for providing translational motion to a belt coupled thereto, wherein the chassis formed of a material having a first CTE, including: first mechanisms for holding a belt pulley axle at a first end, the first mechanisms including mechanisms for fixedly attaching the belt tensioner to the chassis such that the belt is in tension between the belt drive motor and the belt pulley and such that material EC of the first means due to thermal expansion and contraction is diametrically opposed to material EC of the chassis, wherein the first mechanisms are formed of a material having a second CTE and wherein the first CTE and the second CTE are related in proportion to a ratio of a linear distance between the motor and the mechanisms for fixedly attaching the belt tensioner and the distance between the pulley axle and the mechanisms for fixedly attaching the belt tensioner such that EC of each is in substantially identical proportion to the ratio; and a second mechanism for holding the belt pulley axle at a second end, wherein the second mechanism is captured by the first mechanisms and the second mechanism is formed of a material having a third CTE wherein the third CTE is substantially lower than the first CTE and the second CTE such that the second mechanism is substantially unaffected during material EC of the chassis and the second mechanism.
In another basic aspect, the present invention provides an ink-jet printhead scanning carriage drive belt tensioner subsystem for a drive motor and carriage drive belt system, wherein the drive motor is mounted in a first position on a chassis, including: a belt pulley having an axis of rotation; a frame for positioning the belt pulley with respect to the motor, including fastening mechanisms for affixing the frame to the chassis at a second position; and a fork for positioning the belt pulley on the frame such that the belt pulley is between the first position and the second position and the belt is tensioned between the motor and the pulley; the chassis, the frame, and the fork each being fabricated of a material having a differing, compensating, CTE characteristic such that material EC of the chassis and the frame is balanced wherein the axis of rotation remains in a substantially constant position with respect to the first position.
In another basic aspect, the present invention provides a method for maintaining a predetermined tension of a drive belt between a belt drive motor and a belt pulley, including the steps of: affixing the motor to a chassis at a first predetermined position, the mount having a known mount material CTE; affixing the pulley on a frame at a predetermined frame position, the frame having a known frame material CTE; affixing the frame to the chassis at a second predetermined position such that the predetermined tension is established and the pulley is located along a plane between the first predetermined position and the second predetermined position, wherein the length L2 of the frame and the length L3 of the fork is related to the frame material CTE and the fork material CTE, the chassis material CTE and length L1, and the belt CTE and length LBelt, wherein means for affixing the pulley on the frame comprises a material having an affixing means CTE substantially less than the frame material CTE such that substantially no material EC is experienced by the means for affixing during proportional material EC of the mount and the frame.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a scanning carriage for translating at least one ink-jet writing instrument mounted thereon across adjacently positioned print media, including: an apparatus chassis, fabricated of a material having a CTEchassis; a drive motor mounted to the chassis at a first position; a belt pulley mounted on a pulley frame by a pulley fork, wherein the pulley frame is fabricated of a frame material having a CTEframe which is greater than CTEchassis and wherein the pulley fork is fabricated as a fork material having a third CTEfork which is substantially less than the CTEchassis such that there is substantially no center-center change between the motor pulley axis and the turn around pulley axis during thermal excursions wherein the belt material CTE is non-affective as relative to the CTEfork, CTEchassis, CTEframe; and a belt tensioned between the motor and the pulley and having the carriage mounted thereon wherein the pulley frame is affixed to the chassis at a second position with the pulley positioned between the first position and the second position such that a predetermined belt tension is established and maintained regardless of material EC of the chassis and the frame.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a beltriven scanning carriage for translating at least one ink-jet writing instrument mounted thereon across adjacently positioned print media, including: a belt tensioner device, including an adjustably mountable frame for mounting to a chassis, a pulley fork biasingly mounted to the frame, and a belt pulley mounted between the frame and the fork, wherein the frame and fork are co-associated and fabricated of materials each having a complementary CTE to compensate for temperature excursions affecting belt tension.
Some of the advantages of the present invention are:
It maintains a relatively constant belt tension during environmental temperature excursions;
it allows for lower belt tension in a belt drive system, allowing the use of lower cost bush bearings and increasing motor life; and
it provides for increased printer performance at a lower cost.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.